1. Field of the Invention
The present invention relates to primarily an image forming apparatus such as an electrophotography type or electrostatic storage type image forming apparatus such as a copier or printer, which performs density correction of images. The present invention also relates to a density detecting device and density detecting method for detecting density of images.
2. Description of the Related Art
There has been demand for improved stability of image quality in image forming apparatuses which form color images. To deal with this demand, density correction control is performed to correct change in density due to various factors. These factors include secular change in driving members and image forming members of the image forming apparatus from continuous use, change in the environment where the image forming apparatus is installed, change in temperature within the image forming apparatus, and so forth.
An example of density control is disclosed in Japanese Patent Laid-Open No. 2006-145679, where a toner image for density control (hereinafter referred to as “test image”) is formed on a rotation member such as an intermediate transfer belt. The formed test image is detected by a detecting device, configured including a light emitting portion, a light receiving portion which receives normal reflection light that has been irradiated from the light emitting portion, and a light receiving portion which receives diffused reflection light that has been irradiated from the light emitting portion. The detection results detected at the detecting device are used to correct density, by controlling the amount of exposure at the time of forming a latent image, the area ratio when forming the latent image, charging voltage, developing voltage, and so forth, so that the density will be appropriate when forming an image.
However, depending on the state of the image forming apparatus, there may be difference in density according to the position in the main scanning direction of image formation (the longitudinal direction orthogonal to the rotational (circumferential) direction of an image bearing member). This is due to, for example, difference in contact pressure between the developing roller and developer blade in the main scanning direction, variance in photosensitivity and/or surface potential of the photosensitive drum in the main scanning direction, and so forth. Such variance might be able to be handled by forming test images of the same tone at multiple positions in the main scanning direction for example, and averaging the detection results.
However, suppressing variance in this way necessitates sensors to serve as multiple detecting devices to form multiple test images. While the precision of density correction control can be improved by averaging the detection results of the multiple test images, the number of sensors increases as the number of test images increases, so there has been a problem of increased costs.